A known technique for hammering a tube so as to form a pile consists in closing the bottom end of the pile with a plug of strong material, in particular concrete, thereby defining a pile-driving head, and in hammering said head by dropping a mass inside the tube.
Variants of the above technique consist in replacing the plug of strong material with a metal cover (French patent 85 19405). Another known technique is to provide the bottom portion of the tube with a shoulder against which a pointed metal plug bears, which plug is struck by dropping a metal mass (French patent 72 03451).
The major drawbacks of those various systems are the following:
When using a concrete plug, tube penetration is ensured only by friction between the plug and its envelope, and the method is incapable of making piles of great length.
When a cover is used, the pile-driving head cannot be recovered and when a metal plug is used special tools are required to recover it. Unfortunately, it may be necessary to remove the pile-driving head in order to drill through a hard region of ground that cannot be passed through by hammering or in order to perform reconnaissance concerning the ground of a foundation.
All of those methods function poorly or not at all when hammering piles that are inclined, and they work even less well with curved piles as are used quite frequently for guiding the top portions of oil boreholes.
That is why most pile-driving is performed by means of pile-drivers striking a helmet covering the head of such a tube.
The major drawbacks of such head-end striking system are as follows:
The hammering energy required for achieving a determined amount of pile insertion is nearly always greater than the energy that would be required if hammering had been performed at the bottom of the pile.
The thickness of the wall of the pile is determined much more often by the value of the hammering shock force that needs to be withstood than by the final load applied to the pile, thereby giving rise to wasted additional expense.
A head-end hammering system is always very noisy which often means it cannot be used in a built-up area.
When driving very long piles, e.g. for off-shore oil platforms, the head-end hammering technique does not enable the state of the bottom end of the pile to be thoroughly inspected, and when such inspections are performed after the event, it is often observed that a tube is crushed.